Hardening off "water" roots

Do you mean transitioning the root system that grows when one roots a cutting in water?

If so, I would give the roots a hair cut before planting in a porous, coarse textured soilless mix. Don't make the mistake of thinking that the mix needs to be kept soggy to compensate for its loss of a water habitat.

The plant's energy will be spent on making all new roots that are able to grab water from a solid medium rather than dissolved oxygen from a liquid environment.

Sorry for the explanation, if that's not what you meant by 'hardening ' water roots.

I was wondering if an idea I've been batting around would sound feasible or not. Say you have a cutting in a jar of water and after the roots have grown dense enough to look pretty sturdy,you added pea gravel up to the water line,right?
I like the haircut idea! That's good advice that I oft forget,but I can definitely see doing this at this stage of the operation.

So my Idea is to keep an eye on the pea gravel set up and let the water line slowly drop and in theory,the roots should gradually adapt to a non liquid environment.

I'd say no to that. Using pea gravel will not substitute for a planting medium of some sort. It's just another way of growing hydroponically. Roots left high and dry out of water will die.

It's possible to grow plants suspended in the air, but the roots must be misted every few seconds.

Your idea will be much like suspending the roots in mid air only without the nutrient mist. Truly, it's far better for the plant if you plant carefully in a soilless medium. Just be sure that it's highly porous and fast draining.

i've transitioned avocado from water into soilless without a glitch. and it is supposed to be very sensitive. and sulks at the slightest disturbance.
first in water when the roots started going a few inches, i filled the reservoir with glass pebbles. many side roots developed and filled the area. often the water level would drop by half, but there was plenty of moisture trapped between the glass marbles - the roots were living there without the problem: i checked thru the glass to be sure.
after a while i repotted into pure perlite with acrylic wick stuck from the bottom and extended along the side to the top. drenched and let it wick from the bottom, occasionally top watering, since the perlite does dry up on top faster.
after all was well for a while - it went into soilless+40-50% perlite and on self-watering wick again (where it still is, uppotted once since).
the roots cling a lot to perlite - i did not shake it off. watered the day before xfer to make sure perlite was damp but not soggy . then just shook it out sideways, some crumbled away, the rest i kept.
it seems rather involved with many xfers, but the plant did fine and i had minimal root disturbance. no sulking, no leaves hanging down. and of course i bagged it for may be a month after xfer to half-soilles.
i have done the same with zz's - that was the only way they would develop roots and tubers for me after total collapse of both. i mean the water-roots will develop fast, but when i xfered into fine orchid bark or anything else the 'water-roots' would all rot. but done with same stalks as above - they stayed. i still have 2 in pure perlite after 6 mo. and the oldest in half perlite for the past 6-7 mo.

Is this a theoretical question or are there particular cuttings in mind? Babbling in general...

For tough, thicker roots like those in your pic, Petrusha, pulling that plant out eventually (which sounds like the plan Asleep is after) would probably not result in damage. For something with fine, hair-like roots, many of them might be ripped off when trying to remove plant later. If one is going to trim the roots anyway, probably a moot point. Yes 'they' tell us the roots are different (water vs. soil) and I believe them, but still wonder if trimming for the transition is an invitation to bacteria or other pathogens?

I wish someone would make a time-lapse video of a plant replacing water roots with soil roots. Wouldn't it make more sense to us all if we could see it happen? I'm sure the 'water roots' are able to function, though possibly more slowly (?) since I've had very few problems with this transition over the years.

Ideally though, one would not have a cutting in water for a long enough time to worry about these things. As soon as one can see the cutting is inspired to start some roots, it's ready for soil, IMO/E. Very few things I would start in water at this point, after seeing how much better results are that I have by starting directly in soil.

I don't think I've ever done anything but pull the plant out of the water and stick it unceremoniously in 'dirt' when doing such a thing, for so many different plants - unless the mass of roots was just too much for the pot I wanted the plant to go into. For the Coleus cuttings I keep in water for winter, that's how I put them in the yard. Maybe I've been living in a fool's paradise, but I have always been lazy about things that don't seem necessary, and have experienced retrospective irritation with 'instructions' many times. Like when one is hesitant to try something because of the wildly complicated directions. Experience has taught me that if you're too lazy for all that, just don't do it all but try anyway, in your own lazy way. Roses are the main example. I used to read all of this stuff about the trimming, spraying, fretting *needed* to grow them and avoided getting any for years. Then I bought a house with a few old roses that had gotten shaded as nearby trees grew larger. So I dug them up and moved them into more sun and had beautiful roses which I did nothing extra for, as if they were any other shrub. Easy peasy, no packages of stuff or strange routines needed. Not to say that those who fuss over roses are making any mistakes, it's just not for me. Either they'll do well or they won't, I don't want "fussy chem roses."

The things y'all are willing to do impresses me very much, the level of dedication. Thumbs up to any attempts done in any way!! I love hearing everyone's stories, and I'm sure there are plants that would fail to propagate under my watch if I'm not willing to follow actual instructions.

I have moved several cuttings from water rooting to a moist soil medium without too many issues. I then keep the pot in bright shade for a week or two, or if I have done that particular plant before and know its durability, then I might let it sit in morning sun.

As for roses, I have just done this recently with 2 cuttings. The first cutting adapted well and is having new growth from the ground, even though I had thought more new growth would come from the cutting (lots of little buds on it for stems but so far, no stem growth). The second one was recently done but no adverse effects on its existing leaves or stems so far.

One of the things I like to do is to try different methods, since many people have all different instructions for what to do. So if I'm making one cutting, I might as well make 5 or 10, and do different things with them. (I currently have about 10 cuttings of the same rose which were placed directly into a moist soil mix)

i only used the above multi-step xfer for plants extremely sensitive to water-to-medium xfer.
purple,
taking zz's roots as an example of easy to xfer 'water-to-soilless' roots was not good ;) - they rotted for me multiple times without fail, until i did what i outlined above.
nothing else worked. i am still not done with them and it's been over 1 year now. they were one of my favorites and i am fighting for them until last.
it's an exception, not the rule.
i do many other plants water to soilless (usually with 50% perlite) without a thought or problem. like coleus, ficus, ivies, spiders.

I switched a water rooted ZZ & its roots to mix w/ no problem at all. It was a single plantlet of it (w/ a pea sized tuber which sprouted while in the water).

I specifically recall noting that it had BOTH a tuber & a nice size root when I switched it into mix. ZZs as we know can be slow, but I recall no problems & believe I still have the plant several years later. While not large or fast growing, it grows just fine.

we talked about this before in another post. my zz's were much larger then your single leaf plantlets.
so much larger green mass to support.
i have 6 stalks that are 27"-29" each, some had a piece of rhizome, some none at all, some had a little leftover of the tuber.
here's a pic of one set out of water, before perlite xfer.

Thanks for the explanation and further details. My comment about your ZZ roots was only about how stout and sturdy they *look* - like pulling them out of the jar of marbles would not tear them off of the plant. No idea if that's true or not, and if it sounded like I was saying that, I wasn't and appreciate the follow-up. The diameter (and strength) of roots of various plants can be wildly different, the attempted point in regard to adding something like marbles to the jar/bottle of any water cutting. An attractive (IMO) option I've toyed with is gel beads shaped like marbles, available in various colors, or mixed colors. They offer a few more days with the moisture they hold if you're prone to letting allll of the water evaporate. They move around in water more readily than glass marbles. They do get foul after a while though since I didn't rinse them often. Aquarium gravel might appeal to some people.

ZZ is not a plant I've propagated except to divide tubers/rhizomes when repotting. Looks like fun though, I need to play with mine, similar to Sans leaf making fat roots in water. I'm sure I won't be the only one inspired by your cool pics.

There's no denying it's more FUN to watch roots develop in water, if they will.

ah...fun? is in the eyes of the beholder ...i suppose ;)
in my case it was more like dread :) - oh, will they rot yet AGAIN?
the stalks don't develop the tuber right away, the stalk (rachis?) was rotting too for a while, had to cut it sev times.
it was and still is AN ORDEAL!
i was so-o surprised to see how big the tubers got in water, though...totally unexpected. that was fun!
gel-beads sound good - glass marbles are heavy, hard to dump them out without tearing the roots.
next time i'll try to submerge the jar in a sink and roll them out into water or smth.
zz roots are very much like orchid roots. but orchid roots too rot so-o easily! even in bark. i know, i've had orchids for years and still rebloom phals.
at the moment i am rooting a spathe 'domino' teeny-tiny 2" baby shoot that i accidentally pulled off. in reality putting them straight in very damp perlite will save time.
but i just get lazy and do vase-style foliage bouquet and then they root...i am not throwing them out, no way ;).

nah, i don't mind, i was just thinking aloud - trying to reason out why my zz's were so difficult. after being the easiest most florishing plants for 5 years straight. i think they are tired of me :).
and i think asleep is sleeping...;)
oh, but it was actually in his thread about 'what's the largest cutting you have rooted', though :).
but then, again, may be they ARE difficult to xfer, and it was YOU who got lucky? who knows... not many people crazy enough to put zz's in water! i found a single post on some blog and thought to try. but not again...
i need to stick them in a far corner and forget 'bout them...
i've seen posts of people having trouble with spider plants!? ...they grow like weeds for me no matter what i subject them to..i can't kill them if i tried!